The present invention relates to magnetoresistive heads. In particular, the present invention relates to a planarized magnetoresistive head.
Magnetoresistive sensors can be used to sense magnetically encoded information. In operation, a sense current is passed through a magnetoresistive element. Presence of a magnetic field changes the resistance of a magnetoresistive element. Change in voltage drop across the MR element due to the change of the resistance of the MR element can be measured and used to recover magnetically stored information.
These sensors typically comprise a thin strip of NiFe alloy (Permalloy) magnetized along an easy axis of low coercivity. Many other ferromagnetic alloys are also candidates. The strips are usually mounted in the head so that the easy axis is transverse the direction of disc rotation and parallel to the plane of the disc. The magnetic flux from the disc causes rotation of the magnetization vector of the strip, which in turn causes a change in resistivity to a sense current flowing between lateral contacts. The resistivity varies according to difference of the cosine-squared of the angle between the magnetization vector and the current vector with and without the presence of an external magnetic field. Due to this cosine-squared relationship, if the magnetization and current vectors are initially aligned, the initial change in resistivity due to disc magnetic flux is low and unidirectional. Typically, therefore, either the easy axis magnetization vector or the current vector is biased to approximately 45.degree. to increase responsiveness to angular change in the magnetization vector and to linearize the sensor output.
The sensor itself comprises a strip of magnetoresistive material deposited on a magnetic shield layer to form a magnetoresistive element. A series of deposition and etching processes form an active region from a portion of the magnetoresistive element. The active region is the area of the sensor that senses magnetic flux from the magnetic storage medium. An upper magnetic shield acts as a barrier between the sensor and the surface of the magnetic storage medium to prevent flux passing by the sensor from linking back to the sensor. The magnetic shield also serves to protect the sensor from receiving stray magnetic flux from surrounding magnetic storage media.
One problem which affects performance of magnetoresistive heads is the degree to which surfaces in the sensor can be made flat or "planarized." In particular, in prior art sensors, the upper shield has a dip just above the active region of the magnetoresistive element. This degrades off-track performance. Lack of planarization can also cause an electrical short between the magnetoresistive element and the upper shield. This can reduce yields during manufacture. Attempts at planarization of magnetoresistive sensors have focused on etching of a boundary control layer. This can damage the structure of the magnetoresistive head and adversely affect stabilization of the magnetoresistive element.